Various process seals have long been used to secure a tank or other vessel for determining when the level of material in the tank reaches a certain point. Microwave sensors have a significant advantage over other devices in that such devices may be universally employed for detecting the level of almost any solid or liquid, regardless of its conductivity or specific gravity. Moreover, microwave sensors are generally insensitive to dust, vapors, foam layers, or viscous liquid coatings or thin layers of powder on the sides of the vessel.
Several prior patents describing microwave sensing devices are known. For example, U.S. Pat. No. 4,107,993 discloses microwave techniques for detecting the level of a liquid in a vessel. An external chamber constructed of material invisible to microwaves is required, and the system detects amounts of unabsorbed energy to the receiver. Microwave devices of this type experience alignment problems since the transmitter and receiver must be properly positioned with respect to one another. U.S. Pat. Nos. 4,218,678, 4,359,902, and 4,044,355 also disclose microwave sensing devices for determining the level of materials in a tank. Microwave devices which utilize radar technology generally seek to determine the travel time of a signal to the detected material and thence to the receiver. The expense and complexity of these devices limits their practical use to situations in which the actual level of the material in the tank must be determined, as compared to devices which simply determine whether the material has or has not reached a certain level. U.S. Pat. Nos. 3,572,119 and 4,458,530 are similarly directed to devices intended to quantitatively determine the level of liquid in a vessel. A sensor monitors the alteration of the standing wave passing through the liquid to determine the liquid level.
Passing the maximum microwave energy through the process seal is easily accomplished in the absence of condensation or coating build up on the process side of the seal. Numerous references are available that discuss dielectric windows in a lossless, semi-infinite situation. The real problem is to provide a microwave window that minimizes reflection and transmission losses when one surface of the seal is coated with a process material, or when condensation of water occurs on the surface, or when both situations occur.
Additional known patents describe microwave windows. For example, U.S. Pat. No. 3,594,667 describes a microwave window having dielectric variations for the tuning of different resonances. The '667 patent modifies a dielectric window to increase the thickness of the window near the outer periphery and to decrease its thickness at an intermediate radius to lower the frequency of one mode of transmission and to raise the frequency of another mode of transmission. Alternately,. the '667 patent describes the variation of the dielectric constant in the window instead of varying the thickness of the window for achieving the same result. U.S. Pat. Nos. 4,566,321 and 4,670,754 describe a plano-convex microwave lens having no air pockets, voids, discontinuities and being solid in configuration. The lens functions to columnate a signal during its outward passage and to focus the signal on its return passage so as to form an extremely narrow beam of energy; and for causing phase cancellation of the high amplitude signals reflected back from the plano surface of the lens. U.S. Pat. No. 4,765,705 describes a grading surface plasmoncoupler having three layers of dielectric material such that the first layer is of a negative dielectric, the second layer is of a positive dielectric, and the third layer is of a positive dielectric. U.S. Pat. No. 3,818,333 describes a microwave window and antenna apparatus including microwave windows having flat inner end portions which extend parallel to each other and perpendicular to the microwave beam axis.
In all of the applications described, the measurement of the height or level of a material contained in a tank or vessel is tampered by the atmosphere inside the tank. The atmosphere in the tank is such that large amounts of condensed water or liquid can accumulate on the roof and walls of the vessel or tank. Also, large amounts of condensed water or liquid accumulate over the surface of the dielectric window mounted in the tank, for example, in the roof of the tank. Water has a very high dielectric constant and is easily made conductive by dissolved ions. Thus, relatively small amounts of water on a dielectric window can make the window virtually opaque to microwaves. None of the above-discussed references solve, and in most cases their not even discussed, the problem of water accumulating on a dielectric window.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter described for inexpensively yet reliably transmitting microwaves into a tank or other vessel without being inhibited by water or moisture build-up on the dielectric process seal.
It is, therefore, a feature of the present invention to provide a process seal which in normal use provides the most efficient transmission of microwave energy. A feature of the present invention is to provide a process seal which optimizes the foreign build-up of material on the seal.
Another feature of the present invention is to provide a process seal which is not effected by the build-up of condensation on the seal.
Another feature of the present invention is to provide a process seal which is sufficiently strong to withstand the pressures typically associated with such seals.
Yet another feature of the invention is to provide a method of sealing a tank which method is robust to the presence of foreign build-up, condensation and pressure.
Still another feature of the present invention is controlling or manipulating the dielectric of the material associated with the process seal for optimizing the effects of foreign build-up, condensation and pressure.
Another feature of the present invention is to provide a process seal which controls or manipulates the geometric shape of the seal for optimizing the characteristics of foreign build-up, condensation or pressure associated with the seal.
Yet another feature of the present invention is to provide a process seal which controls or manipulates the polarization of the associated microwaves for optimizing the characteristics of foreign build-up, condensation and pressure associated with the seal.
Yet another feature of the present invention is to provide a process seal which controls or manipulates the thickness of the seal for optimizing the characteristics of the seal associated with foreign build-up, condensation and pressure.